In this chapter Darwin outlines his views on variation and its causes. Darwin published On the Origin of Species in 1860, in a time when genetic inheritance wasn't well understood. Previously I have mentioned Mendel, however his work did not gain recognition until 1900. One argument constantly made against Darwin is that because he got some details wrong, his whole theory is wrong. Science is a process, old ideas are replaced with new ideas, incorrect information gets corrected. So Darwin didn't understand genetic inheritance, big deal, he was at least able to recognize the fine details of change over time and selection. Sure, evolution is just a theory, but so is gravity. Scientific theories are shown to be true time and time again, can be replicated, and are observed all over. It's not an abstract concept someone just came up with to explain something, it's a repeatedly observed phenomenon. I cannot understand how some people can selectively pick and choose what science they want to believe in. If you don't believe in evolution, what's the point of getting a flu shot every year? A new flu shot attempts to protect patients from the ever evolving strains of flu.
Okay, I'll get off the soap box and onto the chapter. Variation has been a big theme in Origin thus far, so it's fitting that it gets a chapter. Darwin believed that most variation occurred in the womb. Yes, while developmental interruptions or stresses can result in a deformed offspring, these variations are not genetic and thus not heritable. Darwin admits that there is large ignorance concerning the subject. However, Darwin also says that most variation cannot be accounted for by climate and food.
"When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much to the conditions of life," (Darwin, 179). Please Darwin, be more cryptic. This chapter is confusing, and I think part of that is derived from the fact that modern audiences have the benefit of 150 years of the study of evolution. Some of this stuff is just plain wrong and or confusing.
Darwin spends time talking about use and disuse. I could bore you with the details, but it's not how variation arises. Here's the hypothesis: the use of certain traits makes them more prominent and the disuse of other traits causes them to diminish over generations. Observation wise this kind of makes sense, however natural selection acts to select traits that increase reproductive success, not just survival traits. Also, the use of a trait during an individuals lifetime would not make it more prominent in it's offspring. Instead, directional selection can explain the emphasis on certain traits over time.
Darwin recognized that certain structures in the body lead to change in others. Of course, the body must work together in a complementary way to ensure the greatest chances of survival.
There is a section on the variability of secondary sex characteristics (traits associated with sex that are not directly involved in reproduction) and their similarities in placement between closely related species. This idea could be elaborated into the fact that basic body plans are observed throughout nature, but Darwin doesn't take things so far.
Then of course there's a rant about pigeons -- because what would the Origin be without a tangent or two about Darwin's favorite bird? Yes, he knew a lot about breeding them and they did aid in his observations but really, enough with the pigeons! He's essentially hitting on recessive traits appearing, but does not call it this. He believed that the traits get diluted (and uses some questionable math to boot) over generations and believed that both parents needed to have some of this diluted trait for it to appear in offspring. Yes, and no. There are several ways that traits can appear in progeny, I explained recessive traits in my genetics post, and there are also epistatic genes. Anyway, it's not that the trait gets diluted, it just doesn't get expressed because of the other genes dominating it. Genes aren't cumulative. Darwin considered the appearance of such traits to be a throw back to an ancestral trait.
"To admit [to believing in independent creations of species] is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mocker and deception; I would almost as soon believe with the old and ignorant cosmogonist, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore," (Darwin, 200). Religion is not a subject I choose to tread here on the internet. People are entitled to their beliefs and lack there of as they please. This blog is about Darwin, and I am inferring from the above passage that he may have been able to reconcile the idea of a god with evolution. I personally have not read anything written by him stating his religion. It is, frankly, irrelevant. Religion should not impede scientific observation and theory. Science should not be compromised to religious belief and it is up to scientists to keep their work and spirituality in separate spheres to ensure the greatest quality of research. And that's all I'm going to say about that.
This chapter overall presented several false ideas. Darwin knew there was variation allowing for natural selection, however he had no clue what caused variation. This makes for some confusing reading since he tries to explain variation but then states that his ideas are probably not the main cause of variation.